The Fisher 344 rat seems unique in that it does not gain weight after reaching 9-12 months of age in contrast to most other strains of rat. We propose to study various aspects of cholesterol, triglyceride and phospholipid metabolism in Fisher rats and compare results with those obtained in other rats of the same age. The composition of the lipids of various lipoprotein classes will be compared. Sphingomyelin levels will be measured and metabolism of this phospholipid studied. Cholesterol absorption in young and old Fisher and Sprague Dawley rats will be studied, as will cholesterol synthesis by tissue other than liver.